Steel suitable for razor blades

ABSTRACT

STEEL FOR EDGED TOOLS IN THE FORM OF A THIN STRIP MATERIAL, THE STEEL BEING CHARACTERIZED BY A COMPOSITION EXPRESSED IN WEIGHT PERCENT AS FOLLOWS:   PERCENT CR ---------------------------------- 6-8 C--------------------------------- 0.6-0.8 MO-------------------------------- 0.5-1.5 SI ------------------------------- 0-2 MN ------------------------------- 0-2

July 31, 1973 J, KYLERG I' AL 3,749,568

' STEEL SUITABLE FOR RAZOR BLADES Filed Feb. 16, 1971 v 3 Sheets-Sheet 1 I l l l U 700 200 300 400 500 500 C y 1973 B. J. KYLBERG L 3,749,568

STEEL SUITABLE FOR RAZOR BLADES Filed Feb. 16, 1971 3 Sheets-Sheet 2 Fig. 2B

Fig. 2A

y 1973 B. J. KYLERG AL 3,749,568

STEEL SUITABLE FOR RAZOR BLADES 3 Sheets-Sheet 5 Filed Feb. 16, 1971 Fig. 3A

Fig.3B

United States Patent 3,749,568 STEEL SUITABLE FOR RAZOR BLADES Bengt J. Kylberg and Lars Espen Wold, Hagfors, Sweden, assignors to Uddeholms Aktieboiag, Hagfors, Sweden Filed Feb. 16, 1971, Ser. No. 115,284 Claims priority, application Sweden, Feb. 23, 1970,

2,231 70 Int. Cl. C22c 39/14 U.S. Cl. 75-126 C 3 Claims ABSTRACT OF THE DISCLOSURE Steel for edged tools in the form of a thin strip material, the steel being characterized by a composition expressed in weight percent as follows:

Percent Cr 68 C a 0.6-0.8 Mo 0.5-1 .5 Si 02 Mn 02 The present invention refers to a steel suitable for cutting of relatively soft material, in particular a steel suitable for razor blades and similar edge tools, produced of strip and with a thickness of less than 1 mm., and preferably even less than 0.25 mm.

Razor blades are today generally produced either of pure carbon steel or of martensitic chromium steel, containing 06-10% carbon and 1214% chromium. The advantages of carbon steel consist in its low price and in its hardness after hardening. Its main disadvantages consist in its poor corrosion resistance and its very poor resistance to hardness reduction during tempering. The tempering stability of chromium steel is better but it is also unsatisfactory in the temperature range in which the edges of razor blades are normally provided with a friction reducing coating of plastic material, generally polytetrafiuorethylene (PTFE) or a similar material, which is sintered to the surface of the metal. A primary object of the invention is therefore to produce a steel suitable for razor blades, with better resistance to hardness reduction when tempering in the temperature range from 150-450 C.

According to the current method for the production of razor blades, the material is cooled down to about 70 C. after hardening, in order to reduce the content of residual austenite. This, of course, complicates its production, thus affecting the cost of the end-product. An-

other object of the invention is therefore to make it possible, in principle, to avoid deep cooling after hardening while maintaining a very high degree of hardness. The fact that deep cooling is not required in connection with the new material also causes the effect of variations in hardening conditions on the hardness of the razor blades to be considerably reduced, which is of importance for the stability of the process. If, on the other hand, the new steel is hardened in the same way as the previously utilized 13% chromium steel, i.e., with deep cooling, a considerably increased hardness is achieved, which has a favorable effect on the life of the razor blade or, alternatively, enables the edge angle to be reduced.

The steel in accordance with the invention is more amenable to cold rolling than 13% chromium steel, as a result of which the production is more stable and the productivity is increased.

Owing to the simplified cold-rolling pzocess as well as the intensive nucleation of carbides brought about by the "ice selected alloy composition also a very fine dispersed secondary carbide phase is achieved. This fine-dispersed carbide phase can be brought into solution very quickly, with the result that the time in the hardening furnace can'be reduced to a very considerable extent by comparison with the hardening times required for previous steels. This gives rise to important gains in productivity. At the same time, the hardening graph for the steel in accordance with the invention is relatively level thus giving considerable latitude as regards time variations during austenitizing in conjunction with hardening.

As a rule, the razor blade is tamped out in the unhardened condition, thus producing a strip of razor blades connected by their short ends, which is separated only after hardening. The stampability in the unhardened condition depends mainly on the presence of primary carbides. The concept of stampability also includes the wear of tools. The steel in accordance with the invention can be produced without any primary carbides, as distinct from conventional razor blade steel, where, during normal production, primary carbides are expected to occur if 7 (Percent Cr)+25 (percent C) 30 Finally, the corrosion resistance of the steel, which has a relatively low. content of alloying constituents, is satisfactory for razor blades.

All these elfects are achieved simultaneously with the steel in accordance with the invention which is characterised in having the following composition calculated in percent by weight:

Percent Cr 6-8 C 0.6-0.8 Mo 0.51 .5 Si 02 Mn 02 where molybdenum can be replaced entirely or in part by twice the amount of tungsten as well as with a maximum of 0.5% of each and altogether up to 1% nickel, cobalt, copper, niobium, tantalum, titamium, vanadium, boron and special earth metals, the remainder being in the main only iron with impurities and accessory elements in normal proportions.

The nominal composition of the steel consists preferably of 7% Cr, 0.7% C, and 1.0% M0, the remainder being iron, impurities and accessory elements.

The steel and its properties shall now be explained more closely in the following description in which reference will be made to the attached drawings, of which FIG. 1 shows a tempering diagram for a number of steels of different composition and with diiferent heat treatment.

FIGS. 2A and 2B show hardening curves for a steel in accordance with the invention and for a reference steel, respectively.

FIGS. 3A and 3B show the structures for the same steel prior to hardening, and the diagram in FIG. 4 shows the result of corrosion tests which have been carried out.

Table 1 shows, on the one hand, the compositions according to test analysis for three steels examined, i.e., steels 1-3, the steels 2 and 3 having compositions in accordance with the invention, and on the other hand the nominal compositions of three earlier, commercial steel grades, i.e. steels 4-6. In addition the compositions of two steels 7 and 8 are given, which are taken from reference material for corrosion studies.

TABLE 1 Composition, percent by weight In the test, the steels l-3 were produced in an induction furnace, in charge of 50 kg. The material was forged and hot rolled to 3 x 80 mm. Then the strip was cold rolled in the usual way until its thickness amounted to 0.10 mm. The steels 2 and 3 in accordance with the invention lent themselves to relatively large area reductions between each annealing point.

Table 2 below summarises the hardening behaviour of steels 1-3 during the tests. The table also states the hardening method for the commercial steels 46 in accordance with generally-available brochures. FIG. 1 shows in the form of a diagram how the hardness of the examined and already known materials varies with the temporing temperature.

TABLE 2 .Austenitiz- Steel ing temp., Austenitiz- Curve No. 0. ing time Cooling 2 1,050 30 sec..- To about 7ll O. 2 In water. 5 To about 70 C. 4 To about 70 C. 4 In water. 1 Do. 6 30 sec To room temperature.

After hardening and coating with PTFE at BSD-400 C. the razor blades should have a high hardness combined with a satisfactory toughness to ensure that their life will be as required. The PTFE coating contributes considerably to an increase in shaving comfort, which can be further improved by reducing the edge angle. For a more acute edge however, the material must'be stronger and hence harder. The hardness after tempering at the temperatures which occur during sintering of the PTFE coating onto the surface, which are generally in the range of 300-400 C. (in some cases lower), is for several reasons of great importance.

The diagram in FIG. 1 shows that the highest hardness values after tempering at temperatures between 150 and 450 C. are achieved with steel in accordance with the invention, curves A and B. This effect is particularly notable if the steel is tempered at temperatures within the important range of 350-400 C.

i The materials which come closest to the steel in accordance with the invention, as regards, hardness after tempering within the relevant temperature range, are the steels 4 and 5, curves C and D. The hardness values achieved with them are however considerably lower than with the steel in accordance with the invention. In addition these materials have to be submitted to deep cooling and also the required austenitizing time is longer, amounting to as much as 10 minutes in the case of steel No. 4. Curve F shows that the hardness of steel No. 1, which is of the same type as the commercial steel No. 5, is lowered drastically if deep cooling is omitted. Also with the highly alloyed steel No. 4, the hardness after tempering is significantly reduced if deep cooling is omitted (curve E). Finally, curve G shows the catastrophic reduction in hardness which occurs if carbon steel is tempered, as a result of which it is quite impossible to use it for razor-blades which are to be coated with PTFE by sintering at relatively high temperatures. The high tempering resistance of the material in accordance with the invention is surprising, considering its low chromium content. This is probably due to the fact that secondary hardening occurs owing to the segregation of fine carbides, thus compensating the negative eifect of the reduced chromium content.

The so-called hardness graphs in FIGS. 2A and 2B show the functional connection between the austenitizing temperature and the austenitizing time required to achieve different degrees of hardness. FIG. 2A describes the conditions, after hardening in water, as regards steel specimens (dimensions: 0.1 X 10 x 70 mm.) with the composition in accordance with the invention, and FIG. 2B shows the conditions in the case of steel specimens having the same dimensions but with an alloy composition of known type corresponding to steel No. 5 in Table 1 (test analysis: 14.1% and 0.61% C., remainder iron as well as silicon, manganese and impurities in normal proportions).* After hardening, the specimens made of reference material were cooled down to about -70 C., since deep cooling forms part of the conventional treatment of this commerical razor blade steel, as a result of which the comparison came close to practical operating conditions. Diagram 2A shows that, in the main, the hardness is maintained slightly above 850 HV when hardening the material in accordance with the invention since the austenitizing temperature amounts to about 1,060 C. and the austenitizingtime is 30-1-5 sec. When hardening the already known material with the same austenitizing time and at the austenitizing temperature of 1,100" C. specified for this steel, the hardness, on the other hand, varies to a very considerable extent, i.e.; between about 740 and 770 H! or by about 30 Vickers units, see FIG. 2B. Also with the longer austenitizing time of 45 see. the hardness graph is equally steep, as a result of which the variation in hardness due to variations in austenitizing time is equally great.

The diagrams 2A and 2B also show clearly that with the material in accordance with the invention, FIG. 2A, maximum hardness is achieved at a lower austenitizing temperature than with the previously known material, FIG. 2B, although the latter is subjected to deep cooling as distinct from the material in accordance with the invention. That the material in accordance with the invention need not necessarily be subjected to deep cooling in order to achieve a very high degree of hardness, is due to the fact that its N temperature is as high as about 1 -200 C. while the M temperature is close to room temperature. This means, therefore, that almost complete martensite transformation occurs already as a result of rapid cooling in the cooling block. On the other hand, the known reference material contains a significant proportion of residual austenite after the same treatment. Continued cooling down to about -70 C. is therefore required in order to achieve nearly complete martensite transformation and hence maximum hardness.

The good hardenability of the material in accordance with the invention as compared with the reference material mentioned in the above paragraph, i.e. steel No. 5, is partly due also to its fine-dispersed carbide particles.

*Trauslators note: This is a literal translation of the orig inal Swedish sentence which should presumably mean 14.1% Cr and 0.61% C, remainder iron as well as silicon manganese and impurities in normal proportions.

Hence, the degree of dispersion is higher than 100 particles/ 100 ,um. (with light optical measurement) compared with only 65 particles/ 100 ,urn. in the case of the reference material. The fine carbide dispersion contributes to rapid dissolution and smaller concentration dilferences in the matrix owing to the short difiusion spaces. FIG. 3A and FIG. 3B show the microstructure before hardening of the material in accordance with the invention and of the reference material, respectively.

Corrosion tests were carried out partly in respect of general corrosion, and partly in brine mist adjusted in such a way as to simulate the media in which corrosion of razor blade steel may occur. The general corrosion test comprised electro-chemical measurements. Polarisation curves were plotted for steels 1, 2, 3, 7 and 8 in 0.05 M H 80 nitrogen being injected at 30 C. The polarisation started at 600 mv. SCE (5 min.) and was continued at 50 mv./min. It proved during this test that passivation may occur in the case of steels 2 and 3 in accordance with the invention as also with the high-chromium steel 1, both in the hardened and tempered condition, i.e. a protective layer of oxide was formed. Passivation of steels 7 and 8 was however not possible in this medium.

Specimens of the same steel were then tested in a NaOH solution in the presence of NaCl, a medium which could occur during shaving where the shaving soap acts as an alkaline medium and NaCl is discharged by the skin. The tests were carried out on material in hardened and tempered condition, i.e. the conditions in which the razor blade is used. The mechanically polarized specimens were suspended vertically on plastic coated hooks in the testroom. A brine mist was produced from an aqueous solution of 0.01 M NaOH+0.0017 M NaCl, a one-hour cycle being applied, with 30 minutes mist spraying and 30 minutes rest (without heating in order to cause condensation to form on the specimens). The test period amounted to six days.

The results were evaluated in accordance with the l969-scale for the assessment of painted steel surfaces of the Corrosion Committee of the Swedish Academy of Engineering Sciences:

Corrosion degree 10: Surface appears to be entirely unimpaired Corrosion degree 5: Approximately half the surface is rusty or discoloured by rust Corrosion degree 1: The entire surface is rusty or discoloured by rust The results of the tests are shown in the diagram in FIG. 4. It will be seen that the best value amounting to nearly 10 was obtained with high-chromium steel No. 1, the values for the steels 2 and 3 in accordance with the invention being only insignificantly worse and amounting to 9 or slightly more. Hence, the corrosion resistance may be taken as satisfying the requirements in the case of a material for razor blades. The values for the two reference materials 7 and 8, amounting to approximately 6, were considerably poorer, thus making it necessary to provide them with an anti-corrosion coating in order to produce a corrosion resistance of the same order of magnitude as with steels 1-3.

We claim:

1. Razor blade and similar edge tool in the form of a strip having a thickness of less than 1 mm. for cutting relatively soft materials, which edge tool strip is made from a steel alloy consisting essentially of 6.5-7.5 wt.-percent Cr 0.650.75 wt.-percent C 0.75-1.25 wt.-percent Mo from trace to 2 wt.-percent Si from trace to 2 wt.-percent Mn balance iron and customary impurities.

2. Edge tool according to claim 1, in which the silicon content of the steel alloy amounts to 0.9 wt.-percent.

3. Edge tool according to claim 1, wherein said alloy consists of about 7.0 wt.-percent Cr about 0.7 wt.-percent C about 1.0- wt.-percent Mo from trace to 2 wt.-percent Si from trace to 2 wt.-percent Mn balance iron and customary impurities.

References Cited UNITED STATES PATENTS 1,723,015 8/1929 De Fries -126 C 2,347,375 4/ 1944 Stargardter 75-126 C 1,858,705 5/1932 German 75-126 C 1,911,173 5/1933 Colwell 75-126 C 1,914,990 6/1933 Burr 75-126 C 2,069,260 2/1937 Merter 75-126 C 2,147,122 2/1939 Emmons 75-126 C 2,983,601 5/1961 Fletcher 75-126 C 3,469,972 9/1969 Carlen 75-126 C HY LAND BIZOT, Primary Examiner U.S. Cl. X.R. 75-125, 128 W 

